1. Field of the Invention
The object of the invention is a measurement transducer for the incremental precision measuring of linear quantities, which comprises a measuring rule onto which is reported or etched a measuring scale, and a reading head slidable along said rule. A transducer of this type is generally called an optoelectronic rule or scale.
It is known that in these optoelectronic scales for the incremental measurement of linear quantities, the unitary increments are sensed by reading the moire fringes produced by optical interference--through transparency or reflection--with two graticules of equal physical quantity, one of which is etched on the measuring rule--generally fixed--and the other is carried by a reading head mounted on a cursor usually slidable along the rule.
It is also known that the graticule etched on the measuring rule expands and shrinks on changing of the temperature, according to the coefficient of expansion of the tape material of the rule. This obviously determines, when not affected by other factors, the coefficient of expansion of the scale, which is generally from 10.times.10.sup.-6 to 12.times.10.sup.-6 for steel rules and about 8.times.10.sup.-6 for glass rules.
2. Description of the Prior Art
The linear measurement transducers using a rule in the form of a thin metal tape comprise a support case, generally consisting of an extruded aluminum section, and the metal tape forming the rule, fixed to the case by means of elastic adhesives, elastomer gaskets and/or other elastic means. The European Patent Application No. 83107557, filed by the same Applicant, describes a structure of this type, wherein the thin tape is suspended at its ends, tensioned, so as to form--together with the support case--an integral whole, the entire length of which can be explored by the reading head associated to the cursor.
All these types of transducers are more or less subject to reading errors, according to the thermal expansions of the rule tape and to the dependance thereof on the expansion of the support case. It is known moreover that any external force added to the thermal expansion force of the rule, if axially applied thereon during the change of temperature, causes an increase or reduction of its natural expansion and consequently determines additional changes in the measuring quantity--in the positive or negative sense--according to the entity and direction of the forces which have caused said changes.
Since, generally, the support case of the rule has a higher coefficient of thermal expansion than that of the metal tape, the result is that, on changing of the temperature, the external force produced by the extruded aluminum support case, acting axially, determines changes in the dilatometric behaviour of the thin tape of the rule, thereby reducing the precision and the linearity of the measuring system.
In other cases, where the tape of the rule has a coefficient of thermal expansion higher than its support (i.e. a machine tool member acting as support), the tape--if not pretensioned--becomes slack or taut, when its temperature is increased or decreased.
To overcome this drawback, for example the Heidenhain European Pat. Nos. 167,857 and 126,888 propose a steel tape rule having impressed on it a graticule of smaller size than the real measure. Thereafter the graticule on the rule is enlarged by tensioning the tape until the real measure is achieved.
Nevertheless, the tensioning of the tape for the purpose of enlarging a graticule of smaller size than the real measure determines a loss of linearity in the measure, due to the fact that the elongation remains constant along the tape length only if the cross section (S) of the tape is constant (.DELTA.S=0). (As known, laminated steel tape without size tolerances does not exist).
The lack of linearity caused by the inconstancy of the tape cross section (.DELTA.S&gt;&gt;0) is overcome in the previously cited European Patent application No. 83107557 by impressing the graticule--having the real size of measure--on a pretensioned tape, so as to avoid, when retensioning the tape with the same force, the lack of linearity of the measure. In fact, with this method, the behaviour of the linearity is opposite to that of the previous example, as the loss of linearity is present only in conditions of non-tensioned tape.
So far, attempts have also been made to reduce at least the effects of the thermal expansion of the support case on the thin tape of the rule. For this purpose, corrective systems allowing to partially compensate said effects have been adopted for the transducers with thin etched tape. This has been obtained, for example, by glueing the tape on the support case with highly elastic adhesives and/or by fixing the tape on the support by means of elastomer gaskets, or still better, by fixing the tape to the end of the case by means of springs, after pretensioning the same, as actually described in the aforespecified European Patent Application No. 83107557.